For air, and gaps on the order of a millimeter, the breakdown is roughly a linear function of the gap length: V = 30pd + 1.35 kV, where d is in centimeters, and p is in atmospheres.
(<a href="http://home.earthlink.net/~jimlux/hv/paschen.htm">source</a>)

The partial pressure of propane must exceed its vapor pressure before it will liquify. At 4.03% by volume and a vapor pressure of 122 psi at 70Ã‚Â°F, you can go up to about 122/4.03% = ~3000 psi (204x).

BLB: correct, that's how air can contain moisture at normal temperatures. Even below freezing water has a vapor pressure, which allows ice to sublimate (see what happens to ice cubes that are kept long in the freezer).

Have fans ever been used in hybrids? Pressure shouldn't affect one much, though temperature could be a problem with high mixes. I was planning to include one and have a wiring for it, but I don't have one small enough.
A metallic fan could work even if a computer fn would melt.